Kirk Douglas (b. Issur Danielovitch in 1916) has been around for so long, and for so many decades was such a major Hollywood force, that it's sometimes easy to forget that like every other actor in history, he was once a virtual unknown. Today even a truncated list of his most-celebrated movies reads like the syllabus for a seminar on mid-20th century film: Ace in the Hole, 20,000 Leagues Under the Sea, Lust for Life, Paths of Glory, Spartacus, the noir masterpiece Out of the Past, Lonely Are the Brave, Seven Days in May and so many others.
There's a reason he was honored with a Lifetime Achievement Oscar "for 50 years as a creative and moral force in the motion picture community"; a reason he's won similar awards from the American Film Institute, the National Board of Review, the Berlin Film Festival and others; a reason why—as he celebrates his 100th birthday on Dec. 9, 2016—he remains arguably the greatest living Hollywood legend. And that reason, of course, is that Douglas earned it all (while penning more than a few best-selling memoirs along the way).
But back in 1949, the son of Jewish immigrants from Belarus was just another talented, good-looking actor hoping for a big break. He had some notable roles under his belt already, in strong movies like A Letter to Three Wives and the aforementioned Out of the Past; but it was in the lead role of 1949's unsettling boxing drama, Champion, that Douglas became a star. He played a thoroughly unlikeable fighter named Midge Kelly in the film — a risky move that paid off big-time when he was nominated for a Best Actor Oscar on the strength of his searing performance. (He lost to Broderick Crawford in All the King's Men.)
The rest, as the saying goes, is history. Douglas was a leading box-office draw throughout the 1950s and into the 1960s, while also making a mark as a socially conscious artist. For example, as both the producer and star of the 1960 epic, Spartacus, Douglas insisted that the movie's screenwriter, the blacklisted Dalton Trumbo — a communist from way back — receive credit for his screenplay. That act of conscience, at a time when such stances were almost unknown, is often cited as the beginning of the end for Hollywood's noxious Blacklist Era.
Here, and in acknowledgment of the man's long, full life and his stellar career, LIFE.com pays tribute to Kirk Douglas with a series of photos —none of which ran in LIFE magazine — made when he was on the very cusp of stardom.
A champion, indeed.
Liz Ronk, who edited this gallery, is the Photo Editor for LIFE.com. Follow her on Twitter at @lizabethronk.